The present invention is directed to a process for the removal and recovery of palladium. More specifically, the process is directed to adjusting the pH of a solution comprising soluble palladium and tin, and contacting the solution with an ion exchange resin for the removal and subsequent recovery of palladium metal.
Palladium is widely used as a catalyst to metallize or plate nonconductive, non-metallic materials such as plastics, glasses and ceramics. The products that use a palladium catalyst process to metallize substrates include, among others, printed circuit boards, plastic automotive trim, plastic plumbing fixtures, plastic enclosures to provide RF shielding for electronic devices and the like. The metallization or plating of these various products include similar process steps. The non-conductive, non-metallic surfaces of the substrate are first subjected to cleaning and etching steps to thoroughly clean and roughen up the surface. The substrate is then dipped into a catalyst solution. The catalyst solution is typically an aqueous solution that contains soluble palladium compounds and also usually contains a soluble form of tin. Palladium and tin metal are then selectively deposited onto the surface of the substrate. The palladium deposits serve as a catalyst for plating other metals onto the surface in subsequent processing steps. If an electroless process is utilized, the tin is typically removed from the surface leaving metallic palladium deposits to activate the electroless processing. Typical metals deposited onto the substrate include copper and nickel. The expense of palladium metal limits its use as a catalyst.
Typically, the step immediately following deposition of the palladium metal is a rinse. After rinsing the substrate, there may or may not be an accelerator step that is used to remove any tin that may have been deposited with the palladium metal. If used, as is common in some electroless plating processes, this step is also followed by a rinse of the substrate. Following deposition of the palladium metal onto the substrate, a thin layer of metal is deposited onto the catalyzed surface. As previously noted, this is typically done by an electroless plating process, i.e., plating by chemical reduction rather than by the application of direct electrical current. Alternatively, other plating methods may be employed, such as direct metallization. Direct metallization includes the use of palladium catalyst solution to form the first conductive and catalytic layer in the manner as described above.
Palladium is a very valuable metal and as such, it is desirable to recover the palladium that is present in the rinse or rinses that occur following immersion of the substrate into the palladium catalyst solution. It is also desirable to reclaim rather than to dispose of this metal to the environment in a solid or liquid form, since metals persist in the environment forever, and certain complexes of palladium are known to be carcinogenic.
The palladium catalyst solutions used for metallization contain soluble tin in addition to the palladium ions. Tin is used to stabilize palladium in solution and is normally present in amounts greater than about ten times the amount of soluble palladium. The presence of tin inhibits palladium from spontaneously precipitating out of the solution in metallic form. Thus, tin functions as a stabilizer to prolong the life of the catalyst solution and to maximize the availability of palladium ions for metallization. However, the presence of tin also complicates recovery of palladium from the rinse solutions. Tin is known to precipitate at the pH values commonly used in the rinses. These tin precipitates are difficult to filter and foul most apparatuses employed to recover the palladium. In addition, precipitation of the tin can destabilize the palladium remaining in the rinse solution and consequently, cause the palladium to form a precipitate along with the tin. As such, the amount of palladium available for recovery is diminished. Lowering the pH in the rinses may prevent tin precipitation, but the soluble tin would then interfere with and compete with the recovery of the far more valuable palladium.
Accordingly, there is a need for a process for removing and recovering palladium from solutions. It is desired that the process is relatively simple and highly selective for recovering substantially pure palladium from the solution.
The present invention is directed to a process for the removal and recovery of palladium from a solution. One process includes adjusting the pH of the aqueous solution to a level wherein at least about 90 percent of the palladium remains in solution and greater than 99 percent of the tin in the solution forms a precipitate; and contacting the aqueous solution with an ion exchange resin, wherein the ion exchange resin contains functional groups effective to impart an affinity for palladium ions. The process overcomes the problems associated with the prior art and provides a relatively simple process to selectively recover palladium.
One process for selectively removing palladium ions from a solution comprising palladium ions and tin ions includes the steps of:
a) adjusting the pH of the solution comprising palladium ions and tin ions to a pH range from about 2.0 to about 3.5;
b) removing precipitated tin from the solution; and
c) contacting the solution with an ion exchange resin effective to selectively remove palladium ions from the solution.
Preferably, at least 99 percent of the tin ions in the solution are used to form the tin precipitate. Recovering palladium metal from the ion exchange resin includes heating the resin to a temperature effective to ash the resin or regenerating the resin with a chemical solution to obtain a solution of palladium ions and subsequently reducing the palladium ions to obtain the palladium metal. In the case of regenerating the resin, an additional reducing step includes electrolytically reducing the palladium ions to obtain the palladium metal or chemically reducing the palladium ions to obtain the palladium metal.
Optionally, the step of contacting the aqueous solution with ion exchange resin may include processing the solution in a downflow direction or in an upflow direction through an at least partially fluidized bed of the ion exchange resin wherein the precipitate flows through the fluidized bed and the ion exchange resin selectively removes the palladium ions from the solution.
The inventive process is especially suitable for processing rinse solutions used during a process for plating metal onto a non-conductive material. The rinse solutions are typically aqueous solutions containing soluble palladium and tin compounds. As previously discussed, it is highly desirable to isolate and recover the palladium used in plating operations and the like.
Other embodiments of the invention are contemplated to provide particular features and structural variants of the basic elements. The specific embodiments referred to as well as possible variations and the various features and advantages of the invention will become better understood when considered in connection with the detailed description that follows.
The present invention is directed to a process for recovering palladium from an aqueous solution containing, among other, palladium and tin ions. Advantageously, the inventive process results in the recovery of palladium metal. The process includes the steps of adjusting the pH of the aqueous solution to a specified pH range thereby selectively precipitating the tin and producing a soluble form of palladium. Then, the solution can be either pre-filtered or settled and subsequently processed through an ion exchange resin in a conventional manner, or processed in an upflow direction through an at least partially fluidized bed of ion exchange resin to remove and concentrate the palladium for recovery. Processing the solution through an at least partially fluidized bed of ion exchange resin allows suspended solids to pass through the resin while still allowing removal of the soluble palladium.
In particular, an aqueous palladium/tin solution is chemically treated with an alkali metal hydroxide or hydrochloric acid to a pH range of about 2.0 to about 3.5 and allowed to equilibrate by mixing. Agitation is suspended and the resulting tin precipitate is allowed to settle. This can take place in a variety of different tank configurations or clarification devices as is well known to those skilled in the art. The resulting clarified palladium bearing solution is then transferred to a filter apparatus or an ion exchange vessel containing an ion exchange resin. Preferably, the ion exchange resin is strong base anion resin possessing a quaternary ammonium functional group in the chloride form. A commercially available ion exchange resin suitable for use in the present invention is sold under the trade name IRA 400 available from the Rohm and Haas Company. The solution is then passed through the ion exchange media in a conventional manner or through a partially fluidized bed to selectively remove and concentrate the palladium. The palladium removed from the solution is then recovered from the resin by conventional processes generally known to those skilled in the art. For instance, palladium can be removed by heating the resin to a temperature effective to cause the resin to ash. Once the resin is ashed, the palladium can be recovered by a leaching process. Alternatively, the palladium can be regenerated from the resin with a suitable chemical solution to obtain a solution of palladium containing ions, followed by chemical or electrolytic reduction of palladium to the metallic form so that it can be reclaimed.
The process utilized in the present invention is not intended to be limited to any specific ion exchange resin. Other ion exchange resins with different functional groups may be used and its suitability for use in the present invention will become apparent to those skilled in the art in view of this disclosure. In particular, strongly basic anionic ion exchange resins are preferred wherein the basic groups are quaternary ammonium or phosphonium groups attached to a polymeric backbone. Suitable polymeric backbones include high molecular weight polymers and copolymers, e.g., addition and condensation polymers including polyalkylene, polyester, polycarbonate, polyurethane, formaldehyde resins, etc. Commercially available ion exchange resins include resins based on polyacrylate or styrene-divinyl benzene copolymers. Preferably, the ion exchange resins contain functional groups that exhibit preferential affinity for palladium ions in the pH range specified in the preferred embodiment.
The following examples are detailed description of methods of preparation and use of the process in accordance with the present invention. The detailed examples fall within the scope of, and serve to exemplify, the more generally described methods set forth above. The examples are presented for illustrative purposes only, and are not intended to limit the scope of the invention.